1. Field of the Invention
This invention relates to a color-developer for pressure-sensitive recording sheets. More specifically, the invention relates to an improvement in a phenol-formaldehyde condensate used as a color-developer for pressure-sensitive recording sheets.
2. Description of the Prior Art
Generally, a pressure-sensitive recording sheet material consists of a sheet coated on one surface thereof with microcapsules containing a solution of an electron donating organic compound (a so-called pressure-sensitive dye) in an involatile hydrophobic solvent (to be referred to as a "CB", or Coated Back, sheet) and a second sheet coated on one surface thereof with an aqueous coating composition containing an electron accepting color-developer (to be referred to as a "CF", or Coated Front, sheet). When the two sheets are mated with their coated surfaces facing each other and printing pressure is applied by a ball-point pen, a typewriter or the like, the microcapsules are ruptured and the pressure-sensitive dye solution therein contacts the color-developer to form a colored mark. Accordingly, by using various combinations of the microcapsular layer and the color-developer layer, multiple copies can be produced at a time, or a colored mark can be formed by a single pressure-sensitive recording sheet (to be referred to as an "SC", or self-contained, sheet).
Examples of the pressure-sensitive dye used in such pressure-sensitive recording sheets include triphenylmethane dyes, acylleuco phenothiazine dyes, fluoran dyes, leucoauramine dyes, spiropyran dyes, and the like.
On the other hand, inorganic solid acids such as acid clay and activated clay, phenol-formaldehyde condensates, metal salts of salicyclic acid derivatives, etc. are used as electrophilic color-developers.
Among these color-developers, the phenol-formaldehyde condensates disclosed in U.S. Pat. No. 3,672,935, Japanese Patent Publication No. 18868/1972, etc. are widely used because of their excellent properties such as (1) their excellent color-developing performance, (2) the excellent coatability (or rheological properties) of an aqueous suspension thereof, and (3) the excellent water resistance of sheets coated therewith. Specifically, a p-phenylphenol-formaldehyde condensate is in widespread use as a color-developer for pressure-sensitive recording sheet materials of the multiple sheet type, and a p-alkylphenol/phenol/formaldehyde co-condensate such as the one disclosed in Japanese Laid-Open Patent application No. 84308/1976, as a color-developer for pressure-sensitive recording sheet materials of the single sheet type.
Various defects, however, have been pointed out regarding the p-phenylphenol-formaldehyde condensate as a color-developer for pressure-sensitive recording sheet materials of the multiple sheet type. These defects include the following. (1) Its color-developing ability, especially the rate of color development, is not sufficient. In particular, the initial density of black coloration in reaction with a fluoran-type pressure-sensitive dye such as 3-diethylamino-6-methyl-7-anilinofluoran is low, and at low temperatures its color-developing ability (the rate of color development) is very poor. (2) Marked yellowing occurs during light exposure. (3) Marked yellowing occurs in an atmosphere of an oxidizing gas such as NO.sub.x. (4) It has poor stability to solvent (the phenomenon manifested by a colored image to disappear upon contact with plasticizers such as phthalate esters, gasoline, and oxygen-containing organic solvents such as alcohols and ketones). It is desired therefore to improve this color-developer with regard to these defects.
In the case of the p-alkylphenol/phenol/formaldehyde co-condensate used as a color developer for pressure-sensitive recording sheet materials of the single sheet type, it is also desired to (1) eliminate staining or backgrounding (spontaneous color development) during storage and greatly increase its color-developing performance, and (2) inhibit yellowing of the coated sheets during storage.
There has also been a strong demand for color-developers of high quality which can be used in pressure-sensitive recording sheet materials of both the single and the multiple sheet types.
Various improved techniques have been proposed in an attempt to meet the above desires. For examples, the prior patent documents disclose (1) a combination of (a) a p-substituted difunctional phenol/formaldehyde condensate with (b) kaolin clay and (c) a divalent metal salt such as zinc chloride (see Canadian Pat. No. 842,709, and U.S. Pat. No. 3,516,845); (2) a combination of a p-substituted phenol/formaldehyde polymer (substantially a p-phenylphenol/formaldehyde polymer) with an oil-soluble metal salt such as acetylacetonatozinc or zinc abietate (see U.S. Pat. No. 3,723,156); (3) the use, as a color-developer, of a zinc-modified phenol-formaldehyde novolak polymer obtained by subjecting a p-substituted phenol/formaldehyde polymer (substantially a p-phenylphenol/formaldehyde polymer) to hot-melt reaction with zinc hydroxybenzoate, acetylacetonatozinc, zinc dibenzoate, etc. (see U.S. Pat. Nos. 3,732,120 and 3,737,410); (4) the use, as a color-developer, of a metal-modified phenol-aldehyde novolak resin obtained by subjecting a p-substituted phenol/formaldehyde polymer to hot-melt reaction with zone alkanoates having 1 to 3 carbon atoms (see U.S. Pat. No. 4,025,490); and (5) a polyvalent metal salt of a co-condensate of at least one p-substituted difunctional phenol (specifically, p-phenylphenol, p-tert-octylphenol, p-tert-butylphenol) and at least one trifunctional or higher phenol (specifically, phenol, bisphenol A, bisphenolsulfone) (see U.S. Pat. Nos. 4,216,300 and 4,216,299 and Japanese Laid-Open Patent Publication No. 59519/1978).
Furthermore, U.S. Pat. Nos. 4,165,102 and 4,165,103 disclose a zinc-modified resin obtained by reacting a phenol/formaldehyde novolak resin (substantially a p-alkylphenol/formaldehyde novolak resin) with (A) zinc oxide or zinc carbonate and (B) ammonium benzoate or ammonium formate.
Among the combinations disclosed in Canadian Pat. No. 842,709 and U.S. Pat. No. 3,516,845, the combination of the phenol/formaldehyde polymer, kaolin and zinc chloride has an effect of improving color-developing performance and the light fastness of the resulting colored image, but is not practical because the strong acidity and the water-solubility of zinc chloride (1) markedly thicken an aqueous coating composition containing this combination to cause difficulty of its application, (2) cause a loss of the effect of the coating binder, resulting in a degradation in paper surface properties (adhesion strength), and (3) lead to a danger of leaching out of zinc chloride.
The metal-modified p-phenylphenol/formaldehyde resins disclosed in U.S. Pat. Nos. 3,723,156, 3,732,120, and 3,737,410 are improved in the density and light fastness of a colored image over the unmodified p-phenylphenol/formaldehyde polymer, but exhibit an increased tendency to yellowing in an atmosphere of an oxidizing gas such as NO.sub.x. Hence, these modified resins still fail to meet the desires.
Among the polyvalent metal salts of condensates obtained by reacting at least one p-substituted difunctional phenol and at least one trifunctional phenol with formaldehyde as disclosed in Japanese Laid-Open Patent Publication No. 59519/1978, only a polyvalent metal salt of a co-condensate having p-phenylphenol as a base can exhibit color developing performance comparable to the p-phenylphenol/formaldehyde condensate now in widespread use. Pressure-sensitive recording sheets using these metal salts as color-developers show yellowing behaviors in an oxidizing gas atmosphere or during light exposure to the same or greater extent as or than those containing the p-phenylphenol/formaldehyde condensate as a color developer. Accordingly, the use of this type of color developer has not led to essential improvement of yellowing resistance. Furthermore, polyvalent metal salts of co-condensates having p-alkylphenols as a base have low color-developing performance and their rates of color development do not show sufficient improvement. Thus, these polyvalent metal salts have a low utilitarian value.
The zinc-modified p-octylphenol resins obtained by using zinc oxide or zinc carbonate and ammonium formate or ammonium benzoate, as disclosed in U.S. Pat. Nos. 4,165,102 and 4,165,103 do not give a sufficient density in color development, and a blue colored image obtained by reaction thereof with Crystal Violet Lactone has been found to fade spontaneously in the early stage, giving rise to a problem in practical applications.
Accordingly, these improved color-developers for pressure-sensitive recording sheets disclosed by the prior patent literature still have their own defects, and cannot fully meet the consumer's demands.